This invention relates to a viewing and measuring apparatus, and specifically to an apparatus wherein the measuring point is substantially coincident with the viewing point.
Simple comparators are known in which a measuring scale or grid is superimposed with the object. These devices are relatively coarse scales, and attempting to increase precision and accuracy further by finer division of the scale results in an obscured view of the object. Such a device is not adaptable to the high precision automated measuring instruments to which the present invention is addressed.
Precise and accurate optical measuring devices, per se, are known. For example, U.S. Pat. No. 3,996,463, to Jablonowski, describes the use of grating interferometry to monitor relative movement between a beam of coherent light and a grating. U.S. Pat. No. 3,904,401 to Hock et al, describes an interferometric measuring system without systematic markings on the measuring surface. However, neither system provides for viewing of an object to be measured. Moreover, as will be described, attempting to add viewing capabilities to such a system presents a number of difficulties, particularly if the object is to be superposed with the grating.
Optical measuring systems are known which provide a view of the object being measured. U.S. Pat. No. 3,729,830, to Blachut et al, uses encoding devices that are offset from the viewing ones to allow a clear view of the object. A viewing mark or dot provided by the viewing optics represents the points on the object to which coordinate measurements are related. The accuracy of measurement is limited by the stability of the mechanical linkage between the viewing system and the measuring system, when translated, due, for example, to bearing freeplay and runout, and deformation of linkage elements.
In attempting to provide suitable viewing capabilities for an interferometric system without loss of measurement accuracy, a number of difficulties are encountered. As indicated above, the conventional approach of keeping the viewing and measuring systems physically separate, has accuracy limits due to mechanical guidance errors which are very difficult to improve further. On the other hand, physical superpositioning of the object and grating produces undesirable visible effects in the viewing system. Although grating lines per se, can be made invisible in the viewing system, leakage of the measurement beam into the viewing channel produces distracting pulsing of the beam intensity in the viewing system when translating the object-grating unit. Also, the conventional approach for multiplexing an optical signal for independent utilization, e.g. viewing and measuring, involves the use of broadband beamsplitters which reduces the signal by at least 50% for each pass, significantly reducing optical efficiency. Another approach for multiplexing is using frequency-separated carriers (coloured channels) coupled and uncoupled by dichoic beamsplitters. This provides improved efficiency for one channel but imposes chromatic restrictions. The performance is further degraded if one of the channels is broadband as is typical for a viewing system which the present invention incorporates.